This invention relates to amplitude modulation transmitters and, more particularly, to the high efficiency type of AM transmitters for broadcasting using pulse width or pulse duration modulation. This type of amplitude modulation is described for example in Heising U.S. Pat. No. 1,655,543; Swanson U.S. Pat. No. 3,506,920 and Quidort U.S. Pat. No. 3,943,446.
In such transmitters the audio and subcarrier signals are applied to a pulse width or pulse duration modulator. The resulting pulse width modulated subcarrier signals are applied to amplifiers which increase the power necessary to modulate a high powered carrier signal. Since the efficiency of an amplifier device is maximized at points of maximum current or minimum current, the pulse width modulated subcarrier operates the amplifier in the efficient "on" and "off" states. The subcarrier is then removed via a low pass filter and the DC varying at the audio rate is applied to the RF stage without an expensive transformer to modulate the carrier.
There is a problem with distortion when modulating all the way to zero carrier amplitude (100% negative modulation). This is because such a condition requires that the pulse width in the modulator go to zero. As zero width is approached, the finite switching time of the modulator transistor becomes significant and interferes with the linearity of the system. Distortion associated with narrow width pulses using tubes as the modulators is compensated in Swanson U.S. Pat. No. 2,506,920 and 3,588,744 by amplitude modulating the pulse train.